The present invention relates to improvement in lithium secondary batteries utilizing solid polymer electrolytes.
Lithium secondary batteries recently utilized as main power sources for mobile communication equipment and portable electronic equipment feature high electromotive force and high energy density. Electrolytes used in these lithium secondary batteries are mostly organic electrolyte solutions, each being made by dissolving a lithium salt such as lithium hexafluorophosphate, lithium tetrafluoroboric acid or the like in an organic solvent such as ethylene carbonate, diethyl carbonate or dimethyl carbonate. However, the electrolyte solution of any of the above-mentioned batteries is rich in organic solvent which is connected with danger of ignition, so that a problem of safety is apprehended at all times under the present circumstances. Moreover, the electrolyte solution, which is liquid, has problems of leakage, freezing, evaporation and the others and is devoid of flexibility in battery shape and difficult to reduce its weight.
Techniques for assembling a battery with a solid electrolyte or the like have been conceived as preferred means to circumvent many of the above-mentioned problems connected with the use of the liquid, organic electrolyte solution. Specifically, the means including the use of such electrolytes as an inorganic solid electrolyte, a xe2x80x9cgelledxe2x80x9d solid polymer electrolyte and a xe2x80x9cdryxe2x80x9d solid polymer electrolyte have been actively studied. However, in these means, there reside many problems unique to their respective forms.
For example, such compounds showing excellent ionic conductivity as Li3PO4xe2x80x94Li2Sxe2x80x94SiS2, Li3N and the like have been found for use as inorganic solid electrolytes. These inorganic solid electrolytes themselves are solid, so that unlike the organic electrolyte solution, such phenomena as leakage, freezing and evaporation cannot possibly occur. However, to function as homogeneous, ion conductive layers with high conductivities, thereby to draw out sufficient functions of lithium secondary batteries utilizing these layers, these materials require such processes as grinding, mixing with active material and extremely elaborate molding, causing difficulties different from the solution types.
As disclosed in Japanese Unexamined Patent Publication Nos. 4-306560 (1992) and 7-82450(1995), there is known, as a xe2x80x9cgelledxe2x80x9d polymer electrolyte, an electrolyte in which an organic electrolyte solution with a lithium salt dissolved therein is held by a polar polymer such as polyacrylonitrile. In the case of such a xe2x80x9cgelledxe2x80x9d polyelectrolyte, the electrolyte solution is non-fluidized, so that the danger of leakage has been avoided, and improvement in shape flexibility has been made. However, characteristics of the electrolyte have not outpaced those of the electrolyte solution thus held, and the problem of safety caused by the use of the organic solvent and the problems of freezing and evaporation have not been solved essentially.
On the other hand, there are known, as xe2x80x9cdryxe2x80x9d polymer electrolytes, a solid electrolyte formed by dissolving a lithium salt in polyethylene oxide, a solid electrolyte disclosed in Japanese Unexamined Patent Publication No. 10-204172 (1998), according to which a lithium salt is dissolved in a polymer skeleton of a crosslinked material of a polyether copolymer, and the like. Since no electrolyte in solution form is present in these xe2x80x9cdryxe2x80x9d solid polymer electrolytes, the problems of leakage, freezing and evaporation connected with the above-mentioned solution types can be circumvented. Besides, no organic solvent is contained therein, so that there is increased safety. However, any of the above types of xe2x80x9cdryxe2x80x9d solid polymer electrolytes possesses a decreased cationic transport number because respective counter anions of lithium ions are not immobilized by a polymer chain, thus migrating simultaneously with the lithium ions. In other words, the transport number of cationic lithium decreases, causing rate-determinant mass transfer which problematically results in lack of capability of following high-rate charge-discharge.
To realize a xe2x80x9cdryxe2x80x9d solid polymer electrolyte excellent in ionic conductivity, it is therefore presumably essential for a polymer to immobilize the counter anions, thereby to improve the cationic transport number of the polymer electrolyte. A solid polymer electrolyte disclosed in Japanese Unexamined Patent Publication No. 11-154416 (1999) is presumably one of the ways of turning the conception of such a single-ion conductive mechanism into reality. According to this publication, the disclosed solid electrolyte is presumably a single-ion conductive material made by linking an N group and such an electron-withdrawing group as CF3, SO2CF3, F, Cl, Br, I, SO3CF3, SO2F5 or SO3C2H5 to carbon in the side chain of an isopropylene monomer which forms a polymer, charging a part of the polymer with N+ and then coordinating a mobile Lixe2x88x92 ion thereto to make the lithium ion mobile. However, this single-ion conductive solid polymer electrolyte requires synthesis which is not only complicated but: also time-consuming, so that there have been many problems to be solved before it becomes industrially practical.
Accordingly, development of a novel single-ion conductive solid polymer electrolyte which can readily be synthesized and is excellent both in mass productivity and conductivity has been essential for the realization of a lithium secondary battery excellent in safety and shape flexibility.
The present invention proposes, as a concrete means for solving the foregoing problems, a lithium ion conductive solid polymer electrolyte comprising a polymer having silylamide bonds (Sixe2x80x94Naxe2x80x94Si bonds) in a skeleton thereof and the lithium secondary batteries utilizing the same.
The invention of said novel solid polymer electrolyte is based on the discovery that an organic compound comprising a carbonxe2x80x94carbon double bond and a lithium silylamide compound form a lithium ion conductive solid polymer electrolyte when mixed and polymerized. This synthetic material forms a chemical structure in which a functional group having a silylamide bond, which is a counter anion of a lithium ion, is incorporated in a skeletal polymer chain, so that only the lithium ion becomes mobile, thus contributing to charge transfer. Consequently, a single-ion conductive material possessing an increased cationic transport number is presumably formed.
The polymerization reaction between the polymer comprising the double bond and the lithium silylamide compound can be carried out with extreme ease simply by mixing these components and then keeping them dried. In other words, the present invention can provide innovative improvement not only in mass productivity of a solid polymer electrolyte per se but also in battery manufacturing methods including a method comprising the steps of applying a mixed solution of the above-mentioned components to the respective surfaces of electrodes, joining them together and then drying them, a method of utilizing the mixed solution as an ion-conductive binder when making an active material mix.